As to metal halide lamps used with luminaires for, for instance, outdoor lighting and high ceiling lighting, recent years an improvement in luminous efficiency has been strongly desired from the aspect of energy saving.
In response to such a demand, a certain type of ceramic metal halide lamps has been proposed (see, e.g. Published Japanese translation of a PCT application No. 2000-501563). In a ceramic metal halide lamp of this type, translucent ceramic that withstands a high bulb wall loading, namely withstands use at a high temperature, is used as a material for the envelope of the arc tube. Such translucent ceramic is, for example, made of alumina. The arc tube has an elongated shape (L/D>5, when the internal diameter of the arc tube is D and the length of the space (i.e. distance) between the electrodes is L), and cerium iodide (CeI3) and sodium iodide (NaI) are enclosed therein.
It is said that this ceramic metal halide lamp is capable of achieving extremely high luminous efficiency of 111 lm/W-177 lm/W.
As to conventional metal halide lamps, an arc tube is housed in, for example, a hard-glass outer tube. Here, a quartz-glass sleeve is placed between the outer tube and the arc tube so as to surround the arc tube. The sleeve is provided in order to protect the outer tube from being damaged by broken pieces in the case of rupture of the arc tube (see, e.g. Japanese Laid-Open Patent Application Publication No. H05-258724).
As a matter of course, some conventional metal halide lamps have a structure with no sleeve. However, in such conventional metal halide lamps, fluorocarbon resin coating is applied to the outer tube in order to prevent the outer tube breakage. Alternatively, these conventional metal halide lamps are necessarily used with a luminaire equipped with a front glass so that, in the case of breakage of the outer tube, the broken pieces would not fly off, and thus they are never used with a luminaire having no such a frontal shield facing the floor.
In order to achieve high luminous efficiency, it was attempted to produce a ceramic metal halide lamp as described in the above-mentioned reference (Published Japanese translation of a PCT application No. 2000-501563). A quartz-glass sleeve was placed between the outer tube and the arc tube so as to surround the entire arc tube, as in the above case of the conventional metal halide lamp. When such lamps were prepared and their lamp characteristics were examined, an unexpected problem was posed: due to a rise in the lamp voltage, some of the prepared lamps burned out during the rated life.
With an analysis and examination of the cause of the above problem, the present inventors found traces that, in the burnt-out lamps, the internal surface of the arc tube intensely reacted with the metal halides enclosed in the arc tube. Accordingly, the rise in lamp voltage is thought to be attributable to a significant increase in liberated halides in the arc tube as a result of the reaction between the metal halides and the ceramic forming the envelope of the arc tube.
Then, the cause of the intensive reaction between the metal halides and the ceramic was examined, and the following was found. The ceramic was used to form the envelope because it is a material that is supposed to withstand use at a high temperature. However, the arc tube was made in an elongated shape (e.g. L/D>5) in order to achieve high luminous efficiency, and herewith an arc of the metal halide lamp was formed close to the internal surface of the arc tube during illumination. As a result, the temperature of the ceramic forming the envelope of the arc tube (hereinafter, simply “arc tube temperature”) became a far greater than expected value and reached a temperature at which the ceramic intensely reacts with the enclosed metal halides.
After conducting a further analysis and advancing an investigation, the present inventors also found that the increase in the arc tube temperature was not only attributable to the shape of the arc tube. During illumination, the heat of the arc tube is kept by the sleeve, which accelerates the arc tube temperature increase. As with the conventional metal halide lamp, this has not been acknowledged as a practical problem, and this finding went beyond the expectations of the inventors.
It also became clear that the phenomenon in which the arc tube temperature rises exceptionally high could occur not only when L/D>5, and this phenomenon can be observed when a relational expression of L/D≧4 is satisfied.
In order to solve this problem, simply the outer tube could be made large so that more space is provided between the arc tube and the sleeve. However, this would sacrifice the compactness of the metal halide lamp. Instead, a structure having no sleeve may be adopted. In this case, for example, fluorocarbon resin coating would be applied to the outer tube. However, the fluorocarbon resin coating has limits in its heat resistance, and therefore cannot be applied to all lamps. In the case where even this fluorocarbon resin coating is not applied, the outer tube may possibly break as a result of the arc tube rupture as described above. This was considered to cause a restriction on the applicability of the metal halide lamp to luminaires.